Tunable diode laser absorption spectroscopy (“TDLAS”) has been used to monitor and control processes in a wide variety of combustion and process chambers. TDLAS measurements require a clear line of sight for transmission of a laser beam through the environment being sensed. In many industrial applications, keeping a clear line of sight can be an issue. For instance, in coal-fired power plants, slag and ash can build up in or on sight tube openings to the furnace that provide the necessary line of sight, effectively blocking the laser beam. In steel reheat furnaces or electric arc furnaces, friable refractory and scale can build up in or on sight tubes providing the line of sight access and in electric arc furnaces, molten metal, slag or dust can block the sight tubes. Steam methane reformers, basic oxygen furnaces for steel making, glass furnaces, ethylene cracking furnaces, refinery process furnaces or any other type of industrial furnace, particularly those having a friable refractory, can have similar port blockage issues. All such furnaces and process chambers referred to in this paragraph constitute “process chambers” as used herein.
In addition to port blockage issues, there are other problems that can cause a loss of laser beam transmission in these environments. For instance, in all of the applications listed above, the process chambers or duct walls are mechanically dynamic. Thus, alignment between the laser transmission and receive optics can be difficult to maintain. This can be overcome by the use of automatic alignment systems, for example as described in U.S. Pat. No. 7,248,755. In addition, the particle loading in certain environments, in particular in electric arc furnaces, basic oxygen furnaces and coal-fired furnaces, can be sufficiently high under certain circumstances that no light can be detected (sometimes referred to herein as “high opacity”) leading to a loss of light transmission and the corresponding measurements of such light. In addition, the optical windows separating the TDLAS optics from the process chamber can become fouled, leading to a loss of light transmission. Because the precise cause of a loss of laser beam transmission can be difficult to determine, misdiagnosis of system issues is common and can lead to unnecessary service and maintenance costs. The present disclosure is directed toward overcoming one or more of the problems discussed above.